Tuesday, February 17, 2015

Hey you, with the face! Read this blog about the neurophysiology of prosopagnosia

We’ve all heard the idiom “It’s great to finally put a face to the name!” but for about 2% of the human population this saying means nothing. Put a voice to the name maybe, but a face.. well that’s a different story. Prosopagnosia, also referred to as “face blindness”, is a brain disorder that can be present at birth, called “congenital” prosopagnosia, or develop later in life from neurological damage such as a stroke or traumatic brain damage. Individuals with severe prosopagnosia have difficulty recognizing the faces of even the most familiar individuals. (In this video a woman cannot identify her own mother or even herself when shown pictures.) Prosopagnosia can extend into failure to recognize other’s emotional expressions, age, or gender, but in most cases all other forms of brain functioning are normal.

Developing prosopagnosia late in life seems to be very rare while congenital prosopagnosia is much more common. It is still relatively unknown why certain individuals develop congenital prosopagnosia, however recent evidence suggests that there may even be a genetic link. Dr. Jane Goodall has a mild form of prosopagnosia which she discovered while talking with her videographer about the difficulty of recognizing individual chimps. You can watch her interview here.

So what exactly is going on in the brain that causes people to not recognize even their own face in the mirror? To understand this lets first take a look at the path an image takes from the eye to the brain where it is processed.

When light hits your eyes it causes an electrical signal to travel across three neurons within the eye: first the photoreceptor cell, then the bipolar cell, and finally arriving at the afferent ganglion cell. A cluster of afferent ganglion cells make up the optic nerve which exits the eye and sends information to the brain in the form of more electrical signals called action potentials. However, damage in the signal transduction pathway from the eyes to the brain has not been linked to causing prosopagnosia so lets keep following the electrical signals further into the brain. (Cool note: vision is directly linked to face recognition in wasps)

The axons, or “ends”, of neurons in the optic nerve terminate in four different regions within the brain:

1. Lateral Geniculate Nucleus(LGN) within the thalamus is responsible for visual perception

2. Superior Colliculus in the midbrain is responsible for control of eye movements

Even if you don’t know much about neuroanatomy it’s easy to figure out which of these four brain regions is responsible for perception of faces and objects (hint: it’s the lateral geniculate nucleus).

Pathway of optical processing from the stimulus to the visual cortex.

So, let’s talk about the LGN, shall we? Your brain has two LGN’s, one on the right side which processes information from the left half of your visual field, and one on the left side that processes information from the right half of your visual field. Refer to the diagram above if you get left and right confused easily. Three specialized types of neurons make up the lateral geniculate nucleus and each is responsible for detecting either color, shape, or movement. These neurons pass on the information about color, shape, or movement even further along in the brain into a region called the visual cortex. Termination of these neurons within the visual cortex provides our brains with all the information we need to interpret what we are seeing by releasing different amounts and types of neurotransmitters.

Somewhere in that complex neural pathway is where something goes wrong that causes prosopagnosics to be able to see faces, but not differentiate among them. Dr. Nancy Kanwisher of MIT decided to not only look into this puzzling physiological phenomenon, but to be a subject of study herself! Using functional magnetic resonance imaging (fMRI) she determined the exact location within the brain that is responsible for recognizing faces.

MRI machines allow you to see internal anatomy AND activity by exploiting the increase in local blood flow to brain regions that are active. Dr. Kanwisher spent hours within an MRI machine looking at pictures of peoples faces, then random objects, then peoples faces over and over to determine what region of the brain becomes active when viewing a human face. Over and over again she got the same result (picture below), a tiny spot on the bottom surface of her brain that became active when looking at faces. Then she repeated the study with other people. Same result. But what if that region of the brain recognizes anything human? Or maybe it is responding specifically to the round shape of faces (remember the shape neurons in the LGN?). She repeated the study yet again with pictures that might also be associated with faces, such as the back of someones head and round objects. The same region, now called the fusiform face area, only “light up” when viewing actual faces (including those of animals).

fMRI images of the fusiform face area when viewing pictures of human faces.

By determining the exact region within the brain that is responsible for facial recognition and processing neurologists can now look into exactly what is happening within the neurons of the fusiform face area in people with prosopagnosia. One study on face blind individuals looked at the error rate and amount of time taken to correctly identify an image of a face in which the eyes, nose, and mouth had been inverted. This study revealed that facial processing in prosopagnosics occurs in regions of the temporal cortex that are separate from the visual processing center, and thus rely on sources of information about a person that are not related to the view of the face at all.